As a control circuit configured to control electric power supplied to a load such as an LED, there has conventionally been known, for example, an impedance matching circuit disclosed in U.S. Pat. No. 7,656,103B2, which, using a control signal from a control block, is capable of maintaining a conducting state of a triac in a dimmer by supplying an additional current to the triac when a load current supplied to a load is insufficient.
Incidentally, the current value of feedback current fed back to the control circuit after flowing through the load is unstable. Therefore, when a dimmer current supplied to the dimmer is controlled based on the feedback current, if the current value of the feedback current drops, even for a moment, below a holding current required for holding the triac of the dimmer in a conducting state, the triac turns off so that the on-time of a signal supplying electric power to the load varies per period. As a result, the LED looks like it is flickering.
On the other hand, in order to keep the triac in a conducting state, it may be considered to continuously supply, other than the feedback current, a current greater than or equal to the holding current to the dimmer. However, such a method degrades the power efficiency and thus is not adequate.